Semiconductor materials have many applications in modern technology. In particular, semiconductor materials are useful in the production of microelectronic components such as transistors and diodes. Whilst inorganic semiconductors such as elemental silicon have traditionally been employed in the production of these microelectronic semiconductor components, recently other materials having semiconducting properties have become available and are being increasingly adopted by the microelectronic industry.
One such class of semiconductor materials is that of organic semiconductor materials. These materials are associated with a number of advantages over traditional silicon semiconductors. One such advantage is that organic semiconductor materials can be processed in simpler, less expensive ways. For instance, organic semiconductor materials can be dissolved in certain organic solvents enabling controlled deposition of the semiconductor in the manufacture of microelectronic components. This is advantageous because solution processing is relatively cheap so that a significant saving can be made as compared to the production facilities required to process for example a silicon semiconductor which requires high capital investment in complex vacuum deposition equipment.
Whilst the above-mentioned considerations of how a semiconductor can be processed are important in choosing a semiconductor material from which to produce microelectronic components, a critical characteristic of a semiconductor material is its electronic properties which allow the device to function. Two of the most important characteristics of a semiconductor material which determine its applicability in the production of microelectronic devices are its bulk conductivity and field effect mobility. When a semiconductor is incorporated into a transistor, the bulk conductivity of the semiconductor material determines the conductivity through the semiconductor when the transistor is off (the “off” state). Meanwhile, the field effect mobility determines the conductivity through the semiconductor when a voltage is applied through the gate electrode of the transistor (the “on” state). In terms of the electrical properties of a semiconductor, it is important for proper functioning of a microelectronic device produced using the semiconductor that the on/off ratio (that is, the ratio of the conductivity of the “on” state to the “off” state) is high.
One class of organic semiconductors which fulfils the criteria for applicability in microelectronic components, in particular that of having a high on/off ratio, is the class of polythiophenes. Polythiophenes generally exhibit low bulk conductivities and high field effect mobilities making these materials good candidates for the production of microelectronic components and in particular for the production of thin-film transistors (TFTs).
As explained above, it is highly desirable when processing organic semiconductors to make use of solution processing in which the semiconductor is dissolved in a solvent which is then deposited to produce areas or an extensive layer of the semiconductor. One particularly promising technique for depositing such solutions of semiconductors is ink-jet printing. This is because ink-jet printing conveniently allows relatively precise deposition of liquids onto a substrate in an automated manner. It would be highly desirable to be able to produce microelectronic semiconductor components such as for example TFTs by ink-jet printing a solution of polythiophene onto a suitable substrate.
However, a number of problems are encountered when trying to put this into practice. The key problem is that whilst it is possible to dissolve polythiophenes in solvents commonly used in organic semiconductor processing (e.g. toluene and xylene), the resulting solutions are not ideal for use in ink-jet printing. In order to be able to ink-jet print a solution, it is important that the solution maintains a constant viscosity over time. This is because if the solution does not have a constant and appropriate viscosity, the ink-jet printer cannot maintain the required droplet volume and ejection velocity over time. In addition to this, it is important that a solution to be printed does not contain particulate material above a certain size as such material tends to clog the nozzles of the ink-jet printer. It is also important that the solvent has an appropriate boiling point so that it will quickly evaporate after printing. This is particularly important if multiple overlying layers are to be printed.
In view of these requirements, solutions of polythiophene-based semidonductors in toluene and xylene have not been found to be suitable for ink-jet printing. Although polythiophenes are initially quite soluble in these solvents, there is a strong tendency for gels to form in the solution or for solid material to be precipitated therefrom after just a few days of storage. It is believed that this aggregation of polymer material in the solvent occurs due to interaction between the polythiophene chains. This tendency for the polythiophene chains to interact has been studied in the past because it is this property which results in polythiophenes having semiconducting properties. It is this interaction between adjacent chains which allows charge carriers to travel not only along the delocalised π-electrons along the polymer backbone but also to “jump” from chain to chain particularly when the chains are held close to each other due to strong inter-chain interaction.
For these reasons, there is at present no commercially viable way of ink-jet depositing a polythiophene semiconductor solution on a substrate.
In view of the various deficiencies of known methods of processing polythiophene semiconductor materials, there has been a need for the development of a new method of depositing polythiophene on a substrate. Accordingly, the present inventors set out to develop a new method of doing this which avoids the problems discussed above.